Shape steels having a flange and a web, such as an H-shaped steel, are produced generally through the steps of rough rolling by a breakdown mill, intermediate rolling by a universal rolling mill and finish rolling. Since this method uses horizontal rolls having the same barrel width in the same series, an inner width W.sub.B of an H-shaped steel is constant as shown in FIG. 4(a). When a flange thickness t.sub.F is different, a web height (outer width) W changes with this thickness, the web height becomes different even in the same series, and it is only one set of sizes whose nominal size and web height coincide in each of the standards (JIS, ASTM, BS, DIN, etc). On the other hand, when beams of a building structure are produced by mutually bonding rolled H-shaped steels of several sizes inside the same series, there occurs the disadvantage for the execution because one of the flange outer surfaces is generally registered and deviation twice the difference of the flange thickness occurs in the other. In the case of reinforced concrete building structures, the dimension of the post or the beam is limited by a shell dimension. Therefore, when the conventional rolled H-shaped steels are used, a concrete cladding thickness varies with the size and this is disadvantageous from the aspect of design, too. Therefore, the conventional rolled H-shaped steels are not convenient to use in some cases depending on the applications such as coupling between the post and the beam, between the beam and the beam, between the post and the post, etc, of a building. Therefore, production of rolled H-shaped steels having a constant web height (outer width) W in the same series as shown in FIG. 4(b) has been earnestly desired.
Means for regulating the web height of the H-shaped steels in the same rolling time is described in Japanese Examined Patent Publication (Kokoku) No. 1-47241 and in Japanese Unexamined Patent Publication (Kokai) No. 2-6001. Namely, these references disclose a method of reducing the inner width of the web of an intermediate rolled material after rolling by a rough universal mill at the stage of finish rolling. This is the rolling method characterized in that a finish universal mill equipped with a pair of upper and lower horizontal rolls having a variable barrel width and a pair of right and left vertical rolls is disposed, and when an intermediate rolled material passes through this universal mill, the portion of the intermediate rolled material corresponding to the web is rolled down in the transverse direction by the vertical rolls of the finish universal mill so as to adjust the web height of the intermediate rolled material. This mill is produced by modifying the pair of upper and lower horizontal rolls of a conventional so-called "universal mill" so that the barrel width can be varied, and this method is practical means which makes it possible to adjust the web height of the H-shaped steels by a relatively economical equipment investment.
However, the web portion has a relatively large ratio of the width W.sub.B to the thickness t.sub.W (W.sub.B /t.sub.W : slenderness ratio). Therefore, when the rolling reduction quantity of the web in the transverse direction (web width reduction quantity) is increased, the web undergoes curving or buckling as shown in FIGS. 5(a) and 5(b), and excessive metal occurring due to the reduction of the web width is likely to exist non-uniformly in the proximity of the joint portion (fillet portion) between the web and the flange. In consequence, the local increase of the plate thickness occurs, and non-uniformity of the product plate thickness in the section (plate thickness error .DELTA..sub.t =t.sub.Wmax -t.sub.Wmin) occurs as in the example shown in FIG. 6.
In the extreme case, folding PL of the fillet portion occurs as shown in FIG. 7. Further, because restriction of the rolled material by the vertical rolls proceeds to restriction by the horizontal rolls, the guiding operation of the material to the normal position by the horizontal rolls drops, so that the off-center e of the web (web off-center: .vertline.F.sub.1 -F.sub.2 .vertline./2) is deteriorated as shown in FIG. 8 due to a synergistic operation with buckling of the web. As labor saving and automation on the construction site have made a progress in recent years, dimensional accuracy required for the rolled H-shaped steels as the construction material has become higher, and higher accuracy has been particularly required for an off-center of the web.
The problem of shaping described above can be improved to a certain extent by improving guidance accuracy of the intermediate rolled material to the finish universal mill by the mere contrivance of the guide, and by applying any contrivance to the overall elongation balance by regulating the rolling reduction ratio of the flange at the finish universal mill. However, because the web width reduction quantity cannot be much increased, in practice, by the basic mechanism of shaping, the functions of the barrel width-variable rolls cannot be fully exploited even when such rolls are disposed. Therefore, there remains the problem that not only the assorted production of the web inner widths between a plurality of series cannot be made, and the web heights in the same series cannot be made constant by the same roll set, either, in the series having a large range of flange thicknesses.
To solve the problem described above, the Applicant of the present invention previously proposed a technology in Japanese Unexamined Patent Publication (Kokai) No. 4-100602. This rolling method moves the axes of the vertical rolls of the finish universal mill towards the delivery side of the rolling direction relatively to the axes of the barrel width-variable horizontal rolls, and reduces the inner width of the web while the web of the intermediate rolled material is being restricted by the barrel width-variable horizontal rolls. According to this means, an off-center of the web can be restricted by the web restriction effect of the barrel width-variable horizontal rolls during the web width reduction by the vertical rolls, and the excessive metal occurring due to the reduction of the web width is allowed to fluidize relatively easily in the longitudinal direction by the web elongation promotion effect by rolling of the flange on the delivery side of rolling, so that non-uniformity of the product sheet thickness inside the section can be prevented and eventually, assorted production of web heights, having a large value to a certain extent, can be carried out very accurately. However, this rolling method still involves limits.
In other words, as the web width reduction quantity becomes greater, the region affected by the compressive force of the web is expanded, and this compressive force P becomes greater than a certain limit value as shown in FIG. 9(a). Consequently, web curving WB shown in FIG. 9(b) occurs more on the entry side in the rolling direction than in the influence range WR1 (region represented by a dotted pattern) of the web restriction force by the barrel width-variable horizontal roll 1a (1b). This invites the problems that web curving WB shown in FIG. 9(b) remains even after rolling by the finish universal mill and the web is not guided to the normal rolling position due to this web curving WB and an off-center is likely to develop. By the way, this web curving becomes maximum at the center in the transverse direction due to the influences of web restriction at both end portions of the web by the barrel width-variable horizontal roll and web restriction by the flange.